Multiple control mechanisms appear to exist for the regulation of Class II gene expression. Broadly speaking, these include: 1) those which act at the level of transcription such as lymphokines or transacting factors acting at conserved 5' structural sequences and 2) those which act post-transcriptionally such as at the level of assembly and transport. Very little is currently known about the molecular mechanisms involved in the regulation of Ia gene expression. We propose to study the molecular basis for Class II gene regulation through the use of Ia negative or "null" variant B cells. A number of such Ia variant cell lines have already been produced from a pre-B lymphoma cell line. This group of variant cells was derived from an Abelson-virus transformed pre-B cell. One member of this group contains no detectable mRNA transcripts for any of the Class II genes including the invariant chain (Ii) gene. This cell line can be induced to express Class II molecules by three means: (1) culture with the T-cell derived lymphokine B cell stimulatory factor (BSF-1); (2) culture with antibody to the B cell surface antigen Lyb2; (3) fusion to an Ia+ human B cell, Raji. There are three issues which this variant cell line allows us to study: (1) the site of action of BSF-1 and AlphaLyb2 on Class II genes by examining chromatin structure changes in the presence and absence of these substances or by inducing transfected Class II genes which have deletions in sequences 5' to the promoter; (2) identification of other gene products that are induced by BSF-1 by using techniques of differential hybridization; and (3) identification of the transacting regulatory element in the human fusion partner by first mapping these regulatory factors in chromosomal loss variants and then by sequential transfection of their genes. The analysis of these and other Ia variant cell lines may prove useful in understanding the molecular mechanisms involved in the control of Class II gene expression.